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Friday, February 8, 2008

This is an AstroAlert from Sky & Telescope.

February 8, 2008

Observers throughout the Americas, and even westernmost Europe, have a chance to observe an occultation of an 11.3-magnitude star in Gemini by far-flung 20000 Varuna on Sunday night, February 10-11, 2008. This trans-Neptunian object (TNO) may be as large as 1,000 kilometers across.

Those in South America are especially encouraged to try to observe this event, since they have the highest probability for an occultation. The nominal path misses the Earth to the south, but the real uncertainties in the prediction are hard to assess, as Steve Preston (International Occultation Timing Association) notes. So there's a chance for an event even in North America.

Occultations of stars this bright by such large TNOs are quite rare; so far, none beyond those by Pluto/Charon have been observed. Even if an occultation by Varuna doesn't occur, there could be an occultation by a possible satellite of Varuna.

Closest approach is at 4:26 Universal Time on February 11th in South America, and about 4:30 UT in North America. The formal uncertainty (1 sigma) in the time is about 5 minutes, but you should be prepared for at least a 3-sigma event. I would recommend monitoring/recording the star for at least 20 minutes before and after the predicted time for your station. If an occultation occurs, there will be a 9-magnitude drop lasting about 43 seconds for a central event. The star to be occulted is TYC 1913-00670-1 at right ascension 7h 18m 50.1s, declination +25° 43' 19" (equinox 2000.0). It lies 2.5° SSW of 5th-magnitude Iota (ι) Geminorum and 1.3° WNW of 6th-magnitude 57 Geminorum. Detailed finder charts of different scales are on Preston's website for the event here. His site also has a view of the Earth as seen from Varuna that can be used to estimate your time of closest approach, as well as the altitude of the event above your horizon.

Brian Skiff (Lowell Observatory) notes that the star to be occulted may be slightly fainter than given above, magnitude 11.9 rather than 11.3.

If you are reading an e-mailed copy of this AstroAlert, be sure to look at the online version at SkyandTelescope.com/AstroAlert for possible late updates.

For more about observing occultations in general, check these articles.

Reporting Your Observations

For occultations of stars by asteroids, we have special report forms (.xls versions preferred, but plain-text forms are available as well) at these URLs: